Device for calibrating frequency modulated radar equipment



2,622,241 ENCY 3 Sheets-Sheet 2 KEIZER Mmikuww tau qxcuh MODULATED RADAR EQUIPMENT DEVICE FOR CALIBRATING FREQU Dec. 16, 1952 Filed Feb. 25, 1946 INVENTOR. El lgene U- Kelzer' BY ITTOENEY Dec. 16, 1952 E. o. KEIZER DEVICE FOR CALIBRATINGFREQUENCY MODULATED RADAR EQUIPMENT 3 Sheets-Sheet 5 Filed Feb. 25, 1946 POL 555 a S w P w M llllllllllll 5 III -|||I|||| r m w W W v. w y m w u H a m M P W M Z a. 0 a 5 N 0 W Qv|I| ll 1. w 0 J 0 P r r n y. w N o m w A w w a w M I iifUUW/YT 707744 04/7 07 Patented Dec. 16, 1952 UNITE STTES T FEECE Eugene O. Keizer, Princeton,

Radio Corporation of America,

Delaware N. J., assignor to a corporation of Application February 25, 1946, Serial No. 650,049

2 Claims.

This invention relates to calibrating devices such as are useful in calibrating a frequency modulated radar equipment installed on a moving vehicle, and has for its principal object the provision of an improved calibrating device and method of operation whereby a radar equipment may be calibrated readily and accurately to indicate the distance of the moving vehicle from an object which reflects the transmitted signal back to the radar equipment located on the moving vehicle.

In the calibration of frequency modulated radar equipment, a transmission line connected between the transmitter and receiver of the radar equipment will yield the same effect as a refleeting object located at a distance equal to one-half the electrical length of the transmission line. This effect is useful in calibrating the radar equipment. The maximum length of transmission line that will return a usable signal from the transmitter of the radar equipment to its receiver, however, is limited due to attenuation and becomes shorter as the operating frequency of the radar equipment increases. This means that for some types of radar equipments the operation at maximum rated distances cannot be calibrated, and for other types of radar equipment the overall operation cannot be checked easily anywhere within the distance range over which the equipment operates. This difficulty has resulted in calibration procedures which are indirect or have involved extrapolation to cover some of the dis-- tances over which the equipment operates. Such procedures are somewhat involved and leave something to be desired from the viewpoint of accuracy.

In accordance with the present invention, these difficulties are avoided by the provision of an improved calibrating device and method of operation whereby a synthetic audio frequency signal dependent on the frequency modulated output of the radar equipment and on signals developed by the calibrating device is applied to the receiver of the radar equipment and utilized to provide the required calibration. Developed within the calibrating device are signals equally spaced in frequency over a frequency band which may be adjusted to include the frequency at which the equipment to be calibrated is operating: These signals will be referred to as side frequencies although they are not necessarily generated as true side frequencies in the usual sense. Thus when a frequency modulated signal derived from the transmitter of the radar equipment to be calibrated is mixed with the band of signals developed by the calibrating device a number of beat notes of varying frequency will be produced. These beat notes are amplified, limited to a maximum level, passed through a selective filter system called a shaper, and are then rectified with a peak detector to produce an audio signal. This rectified audio signal is applied to the audio amplifier and counters of the receiver of the radar equipment. With this procedure, the audio signal used in the calibration is dependent on the frequency modulation characteristics of the radar equipment so that ready and accurate calibration over the entire operating range is realized.

Important objects of the present invention are the provision of an improved calibrating device which does not involve the use of the transmission lines heretofore proposed for this purpose; and the provision of a calibrating device which is accurate and readily operated without extrapolation.

The invention will be better understood from the following description considered in connection with the accompanying drawings and its scope is indicated by the appended claims.

Referring to the drawings:

Figure 1 is a block diagram indicating the relation between the calibrating device which is shown at the left and the equipment to be calibrated which is shown at the right,

Figure 2 is a wiring diagram of one type of the RF oscillator of Figure 1,

Figure 3 is a wiring diagram of the video amplifier, limiter, shaper and rectifier of Figure 1,

Figures 4 and 5 are explanatory diagrams relating to the operation of the improved calihrating device, and

Figures 6 and '7 illustrate the details of means which may be provided in the calibrating device for taking into account the speed of a vehicle in which the equipment to be calibrated is to be operated.

Fig. 1 shows the equipment to be calibrated as including an audio frequency amplifier and counter element I I, a radio frequency element l2 and an element l3 for indicating the count by which the distance from the vehicle to the refleeting object is indicated. Other elements common to a radar are not shown for the reason that they are not essential to an understanding of the present invention. It is, of course, apparent that the radar equipment includes, among other things, transmitting and receiving antennae and that such antennae may be utilized to transmit energy between the calibrating device and the equipment to be calibrated instead of the illustrated conductive connection.

The calibrating device M includes an oscillation generator l5, a detector IS, an amplifier [1, a limiter [8, a wave shaper I9, a rectifier 20, a frequency modulator 2| which is connected to a frequency modulation monitor or indicator 22, and an amplitude or pulse modulator 23 which is connected to a pulse rate indicator or controller 24. The frequency modulator 2| and monitor or indicator 22 are used only where the speed of the vehicle must be taken into account in the calibration. Frequency modulated potential derived from the radio frequency element i2 is modulated by the side frequencies delivered from the generator l and the resulting beat notes are detected by the rectifier 20. The resulting audio potential is applied to the audio frequency and counter element II and is indicated by the indicator I3. Since the pulse rate of the calibrating device is known, the control elements of the equipment to be calibrated are readily adjusted to provide indications in terms of the distance between the equipment to be calibrated and a reflecting object.

Figure 2 shows the connections of one form of the side frequency generator l5. This form operates as a self-quenched oscillator, thus producing pulses depending upon the quenching rate. It is well known that such an oscillator produces a large number of side frequencies spaced in frequency by the frequency of the quench-rate. It includes a triode 25. having an anode 2G, a grid 21, a cathode 28 and a cathode heater 2.9. Current is supplied to the heater 29 from a 6.3 volt terminal 30 which is bypassed to ground by a capacitor 3! and is connected to the heater through coils 32 and 33. The oathode 28 is grounded through a coil 34. Bias potential is applied to the grid 21 through an adjustable 250,000 ohm resistor and a resistor 86 of 33,000 ohms. Potential is applied to the anode 26 from a {B terminal 3! which is bypassed to ground by av capacitor 38 and is connected to the anode through a coil 39. The grid 21 is coupled through a capacitor 40 to the upper terminal of the coil 39.

When the side frequency generator of Fig. 2 is a self-pulsed radio frequency oscillator, the external pulse modulator 23 is not required. The pulse modulation of the radio frequency oscillator functions to produce side frequencies spaced in frequency by the pulsing rate. This is seen by making an analysis of uniformly repeated pulses, which shows they are the equivalent of the combination of a sine wave of frequency equal to the pulse rate, plus a sine wave of approximately equal amplitude for each harmonic of the pulse rate. Thus the output of the self-pulsed oscillator of Figure 2 will consist of a carrier and a number of side-frequencies centered about it resulting in a band of signals equally spaced in frequency and approximately of equal amplitude. The fundamental frequencyof the generator can be the same as that of the equipment to be calibrated or can be lower in which case a harmonic of the generator frequency must be of approximately the same frequency as the fundamental frequency of the unit to be calibrated. The frequency to which the generator is set is not critical and the same is true of its stability. In order to. eliminate interaction between the calibrating device and the equipment to be calibrated, the use of a hormonie of the generator is preferred. Use of such a harmonic does not alter the spacing of the side bands.

Since the pulsing rate determines the distance portion of the calibration of the output signal of the equipment to be calibrated, it must be accurately known. This can be done by controlling it with a signal of known frequency injected into the grid circuit at the terminals 50 or by monitoring it by the pulse rate control device 24. To represent a given distance, for any type of frequency modulation radar equipment, the pulsing rate is of course made such that the time interval between pulses is equal to the time interval between the transmission and return of a signal from a reflecting object located at that. distance. In actual use of the equipment to be calibrated with such a reflecting object the frequency of the audio signal appearing in the counter circuits. is the change in transmitter frequency during the time interval between transmission and return of the reflected signal, which may be expressed as: rate of change of transmitter frequency multiplied by time interval during transmission. When using the calibrating device one cycle of audio signal is supplied to the counter circuits of the equipment to. be calibrated when the transmitter of the latter equipment changes in frequency by the amount separating the side frequencies of the calibrating device, and the audio frequency is: rate of change transmitter frequency divided by side frequency separation.

Since the side frequency separation is the pulse rate which is the reciprocal of the interval between pulses, the audio frequency supplied by the calibrating device is: rate of change of transmitter frequency multiplied by time interval between pulses. Thus a given time interval between pulses when using the calibrating device will produce the. same audio signal in the counter circuits of the equipment to be calibrated as will the reflected signal in actual use from an object or target when the total time interval of transmission is the same.

In calibration the procedure is to: 1) couple a small, portion of the frequency-modulated output of the equipment to be calibrated to the calibrating device, (2) set the time interval between pulses equal to the time interval required for a radio wave to travel to and from a given distance, (3) couple the audio signal from the calibrating device'to the equipment to be calibrated, and (4) make the proper adjustments on the equipment to be calibrated for the givendistance chosen in (2).

The output of the generator I 5 (Fig. 1.) and a small portion of the output from the equipment to be calibrated are coupled into the detector l6 which may be a crystal rectifier. With theseconnections, the output of an equipment to be calibrated sweeps across the multiple side frequencies of the generator to produce a varying-frequency beat note in the detector output for each side frequency. These beatnotes are then limited in maximum amplitude and passed through a, selective filter called a shaper." The purpose of this shaper is to form. the response to a beatnote as it changes in frequency from. zero to that of the pulse rate into approximately that of mixirnum response at a frequency equal to half that of the pulse rate and minimum response at one cycle of a sine wave withv zerofrequency and at a frequency equal to the pulse rate. This is illustrated in Figure 4. The output of the shaper is detected to produce the audio output which will be used for the counter circuits of the equipment to be calibrated. As the beatnote from a given sideband varies through zero frequency the total signal from the rectifier will appear as illutrated in Figure 4. However, the beatnotes from adjacent side frequencies will also be present and their response must be added to obtain the total signal from the rectifier. As the frequency of the transmitter of the equipment to be calibrated varies across the band occupied by the side frequencies the resultant total output with respect to frequency is shown in Figure 5.

The resultant total output of the detector i6 is applied to the input terminals 5l52 (Fig. 3) of the channel including the amplifier 11, the limiter Hi, the shaper i9 and the rectifier 2D.

The amplifier ll includes two identical stages which are coupled together through a 100 microfarad capacitor 53 and a 16 microfarad capacitor 54. Thus the first stage includes a pentode 55 which has a 100,000 ohm grid leak resistor connected across its input terminals 5|52, a 200 ohm cathode lead resistor 5'! shunted by a .05 microfarad capacitor 58, a .05 microfarad capacitor 59 connected between its screen grid and ground, a 160,000 ohm resistor through which positive potential is applied to its screen grid, and a 51,000 ohm resistor through which +B potential is applied to its anode. The second amplifier stage is identical with the first and has its output coupled to the limiter l 8 through a 100 microfarad capacitor 02 and a 16 microfarad capacitor 54.

Potential is applied from a +270 volt +B terminal 64 (1) through a 510 ohm resistor 55 to the first stage, (2) through the resistor 65 and a 1000 ohm resistor 60 to the second stage, (3) through the resistors 65, 66 and a 68,000 ohm resistor 6 to the limiter l8, and (4) through a 12,000 ohm resistor 58 to the shaper I9. Each tube of the channel, with the exception of the rectifier, is a pentode and the cathodes of all these pentodes are connected in parallel with one side of each cathode grounded.

The limiter [8 includes a pentode 60 which has a 12,000 ohm grid input resistor 70, a 15,000

ohm grid leak resistor 'H, and a .05 microfarad capacitor connected between its cathode and screen grid. Positive potential is applied to its screen grid and anode respectively through a 47,000 resistor '52 and a 27,000 ohm resistor '53. With these circuit constants, the limiter functions to insure that all the useful beat notes delivered from the detector I5 and amplified by the amplifier ii are made to have the same amplitude or level when they reach the shaper 19 to which the limiter is coupled through an 8 microfarad capacitor 14, a 270 microfarad capacitor 15, a 47,000 ohm resistor 16 and a 12,000 ohm resistor 11.

These beat notes of the same level pass from the limiter into the shaper [9 which produces an audio frequency cycle for each side frequency of the generator swept over by the output of the equipment being calibrated.

The shaper 19 includes a pentode l8 which has its control grid connected to an adjustable terminal of a resistor 18, its cathode grounded through a resistor 79, its screen grid coupled to its cathode through a .05 ohm resistor 80, and its operating potential applied to its anode through a coil 8| and to its screen grid through a 27,000 ohm resistor 82. The input resistor 18 is shunted by a coil 83 connected to a movable contact 84 which is moved to connect in series with the coil 83 a 33 microfarad capacitor 85, an 82 microfarad capacitor 86, a 200 microfarad capacitor 81, a 510 microfarad capacitor 88, an 820 microfarad capacitor 89 or a 2400 microfarad capacitor 90. The output coil 81 is shunted by a slider 9| which is moved to connect it in series with a 6800 ohm resistor 92 shunted by a 220 microfarad capacitor 93, a 6800 ohm resistor 94 shunted by a 470 microfarad capacitor 95, a 47,000 ohm resistor 96 shunted by a 1100 microfarad capacitor 91, a 3300 ohm resistor 98 shunted by a 2400 microfarad capacitor 99, a 1500 ohm resistor I00 shunted by a 5700 microfarad capacitor 101 or an 1300 ohm resistor I02 shunted by a .01 microfarad capacitor I 03. The sliders or movable contact members 84 and Si may be arranged to be moved together as indicated by the dotted lines.

By means of the selector switch Hit, the shaper circuits are peaked at a frequency equal to half the separation of the side frequencies, having a low-frequency cut and a rejection at twice its peak response frequency. The proper setting of the selector switch I04 for determining the frequency response of the shaper circuits is not critical so that it may be ganged with the pulse rate control of the side band generator for single control operation.

The output of the shaper I9 is applied through a 570 microfarad coupling capacitor 405 to a rectifier I06 which delivers through a filter network and the output terminals I0'il08 an audio frequency potential having one cycle for each side frequency of the generator.

Fig. 4 shows the form of the signal as it appears respectively at the output of the limiter, at the output of the shaper, and at the output of the rectifier. From what has been said, it follows that one cycle of audio frequency potential is produced at the output of the rectifier for each side frequency of the generator and that each cycle of the audio frequency potential represents a unit of the equipment to be distance between the calibrated and the reflecting object. These cycles are counted and indicated by the counter of the equipment to be calibrated so that the calibration is dependent upon the frequency modulation characteristics of the particular equipment being calibrated and a high degree of accuracy in the operation of the calibrated equipment is insured.

In cases where the speed of the vehicle on which the equipment to be calibrated is to be located must be taken into account in the calibration, this is effected by applying a sawtooth frequency modulation to the side band generator. This causes the side frequencies to shift in frequency in one direction except for the sharp return snaps of the sawtooth wave. When the transmitter frequency of the equipment to be calibrated sweeps in the same direction as the side frequencies are moving, it overtakes them more slowly than when it sweeps in the opposite direction. Thus two audio frequency signals are produced, the one during the transmitter sweep in increasing frequency direction and the other during the transmitter sweep in decreasing frequency direction. This result is similar to that found in actual use of the radar equipment where there is relative motion between it and a reflecting target.

The required sawtooth frequency modulation 7 may be produced by a rotating capacitor. The capacitor l09-Hlli-| H of Figs. 6 and 7 is suitable for this purpose. It includes a plate I09 which is connected to the grid 21 of the triode 25 (Fig. 2), a plate Hill which is connected to the anode 26 of this same triode, and a rotor III which is driven by a, motor H2. The shape of the rotor III is shown more clearly in Fig. 7.

To provide a good sawtooth characteristic the face of the rotor HIv was found to require the shape shown in Fig. 7. The plates I09 and H are thin strips placed close to the rotor II I and at right angles to its direction of rotation so that the amount of frequency modulation for each rotation of the rotor l l l is fixed.

Byincreasing the rotational speed of the rotor Ill, the amount of effective speed in the calibrating signal can be increased and vice. versa. To monitor the rotational speed of the capacitor, a small iron core pickup coil is mounted alongside the driving motor, with a shunt capacitor to resonate the response of the coil and to bypass extraneous high frequencies. The output of this coil is used to operate an indicator or may be monitored by an oscillograph. and external audio generator. Because the amount of speed introduced into the calibration depends on the characteristics of the individual sawtooth generator, the calibrating device itself must be calibrated in order to provide speed information. Such calibration, of course, is not required where only distance or range information is required,

In cases where conductive connections between the calibrating device and the equipment to be calibrated are inconvenient or impossible, the required signals may be transmitted between the two over wireless, channels. Thus the audio signal may function to amplitude modulate an oscillator which transmits back to the radio receiver of the calibrated equipment. If such, an oscillator is tuned somewhat outside the frequency range of the transmitter of the equipment to be calibrated, extraneous beat notes are avoided. Wireless return of the audio signal to the calibrated equipment, along with the need for only a small amount of coupling to the transmitter of thisv equipment has the advantage that it makes it possible to calibrate an equipment without removing it from a vehicle or other support on which it is mounted.

What the present invention provides is an improvedv calibrating device and method of operationwherebya radar equipment, an altimeter or the like may be calibrated accurately to indicate the distance between it and an object from which waves are reflected backto it, such calibration being effected by sweeping. the frequency modu- 8 lated output of the equipment to be. calibrated over a series of side frequencies produced by the calibrating device and making available. an audio frequency signal which represents the desired distance and is counted and indicated by the calibrating device.

I claim as my invention:

1. In a device for calibrating a radar equipment having a frequency modulated output and having an indicator which is to be calibrated to indicate the distance from said equipment to an object from which said output is reflected, the combination of means for producing pulses at a predetermined rate, means for adjusting said rate to selected values, means for indicating said values, means for mixing said pulses with the output of said radar equipment to produce heat notes occurring at said rate, means responsive to said beat notes for producing an audio frequency signal, and means for applying said audio signal to said indicator which is to be calibrated.

2. In a device for calibrating a radar equipment having a frequency modulated output and having an indicator which is to be calibrated to indicate the distance from said equipment to an object from which said output is reflected, the combination of means for producing pulses at a predetermined rate, means for adjusting said rate to selected values, means for indicating said values, means for mixing said pulses with the output of said radar equipment to produce beat notes occurring at said rate, means responsive to said beat notes for producing an audio frequency signal, means for applying said audio signal to said indicator which is to be calibrated, and means for adjusting said rate to compensate the eifect of movement of said equipment with re spect to said object.

EUGENE O. KEIZER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re.21,955 ChaiTee Nev. 25, 1941 2,176,742 La Pierre Oct. 1'7, 1939 2,248,599 Alexanderson July 8, 1941 2,324,077 Goodale et al. July 13, 1943 2,408,742 Eaton Oct. 8, 1946 2,418,538 Yetter Apr. 8, 1947 2,420,211 Tourshou May 6, 1947 2,421,016 Deloraine et a1. May 27, 1947 2,422,064 Anderson et al. June 10, 1947 2,423,644 Evans July 8, 1947 2,433,804 Wolff Dec. 30, 1947 

